Concealed random allocation protects against bias in clinical trials

Randomised controlled trials use chance to assign participants to groups. The unpredictability of the process should prevent differences in the prognosis of the comparison groups, provided enough patients are randomised. On p 1185 Kunz and Oxman's systematic review provides evidence to support the use of concealed allocation in randomised controlled trials. Not using concealed random allocation was associated with overestimates of effectiveness of 150%, underestimates of 90%, reversal of the estimated effect, or no difference. On average it produced overestimates of effectiveness due to a poor prognosis in non-randomly selected control groups compared with randomly selected control groups. The effect of not concealing allocation can be as large as or larger than the effects of interventions. It is a paradox that the introduction of unpredictability by using random allocation is the best protection against the unpredictability of the extent to which the results of non-randomised clinical trials may be biased.


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The unpredictability paradox: review of empirical comparisons of randomised and non-randomised clinical trials

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